1. Field of the Invention
The present invention relates to methods of inducing a permanent refractive index pattern, or diffraction grating, in optical devices involving low phonon energy materials, through the use of interfering ultrashort and intense laser pulses.
2. Description of the Related Art
The traditional method of writing Bragg gratings in germanosilicate waveguides relies on the use of UV lasers and interferometry techniques that induce a periodic refractive index change in the waveguide. The spatial modulation of the refractive index change along the waveguide is generated using the two-beam interference technique as disclosed in U.S. Pat. No. 4,807,950 by Glenn et al. A more convenient method used to write Bragg gratings in germanosilicate waveguides relies on the phase-mask technique, as disclosed in U.S. Pat. No. 5,367,588 by Hill et al. In this case, a single UV light beam is used to generate the interference pattern through a diffractive element known as a phase mask. Unfortunately, this method was reported to induce a weak refractive index change in fluoride glass-based waveguides, as reported by Williams et al. in J. Lightwave Technol. 15, 1357 (1997).
Taunay et al. first reported a weak permanent refractive index change in Ce3+—doped fluoride-based glass through UV exposure at 245 nm in Opt. Lett. 19, 1269 (1994). However, no significant refractive index change has been reported yet in undoped fluoride glasses using the same method. The use of the two-beam interference method was disclosed to write Bragg gratings in chalcogenide and chalcohalide-based infrared transmitting optical fibers as disclosed in U.S. Pat. No. 6,195,483 by Moon et al. In this prior art, an exposure time of about 3 minutes was necessary to saturate the refractive index change of chalcogenide-based fibers.
Another method of writing permanent Bragg gratings in waveguides is based on the use of interfering high intensity UV beams in order to locally damage the glass to create the refractive index change pattern as disclosed by Askins et al. in U.S. Pat. No. 5,400,422. The drawback of this method is that the refractive index change arises from periodic localized damages induced at the core-cladding interface of the fiber. The process is then closely dependent of the core and cladding glass compositions. The resulting gratings also present poor spectral quality since the refractive index change is only affecting a fraction of the propagating mode to be reflected. The 193 nm radiation was also used in the first attempt to produce refractive index changes in undoped fluoride glasses. Sramek et al. (J. Non-Cryst. Solids 277, 39 (2000)) observed that the fluorozirconate glasses photosensitivity was the result of a glass expansion under such 193 nm light exposure. Following on this work, Zeller et al. (J. Lightwave Technol. 23, 624 (2005)), reported refractive index changes of about 2×10−4 in fluorozirco-aluminate (FZA) and about 2×10−6 in fluoroaluminate (FA) and fluorozirconate (FZ). However, the refractive index change was strongly dependent on glass composition and did not appear to be applicable to the glass compositions that can be drawn into optical fibers. In fact, since it is related to a glass expansion instead of a glass compaction as in the case of silica glasses, the mechanism of refractive index change in fluoride glasses thus appears to rely on a different glass rearrangement. The same glass expansion was also observed in chalcogenide glasses under sub-bandgap illumination and was used to produce convex microlenses as reported by Hisakuni et al. in Opt. Lett. 20, 958, (1995).
A relatively new approach to photosensitivity based on the nonlinear absorption of high-intensity infrared radiation in the femtosecond pulse duration regime was reported by Davis et al. in Opt. Lett. 21, 1729, (1996) to induce wave guiding structures in glasses. Although the precise physical process responsible for the femtosecond pulse induced refractive index change is not fully understood and appears to depend on the glass itself, it apparently relies on the creation of localized plasma within the glass. In order to reach the appropriate plasma density, the writing beam must reach some critical intensity value, which is depending on various factors, including pulse duration and energy as well as focusing conditions. This promising approach to glass photosensitivity was disclosed by Miura et al. in U.S. Pat. No. 5,978,538 and was found to be useful to write waveguides in bulk silica and fluoride glasses. The fluoride glass composition used as an example in this patent is a fluorozirconate glass (ZrF4—BaF2—LaF3—AlF3—NaF), which is a common glass composition used in optical fiber fabrication. This approach was also used in relation to the creation of an interference femtosecond fringe pattern obtained with two spatially correlated beams by Maznev et al. in U.S. Pat. No. 6,204,926. This approach was further extended and disclosed by Miller et al. in U.S. Pat. No. 6,297,894 where the periodic refractive index change is now obtained based on the use of a diffractive element. An alternative version of Miller's technique was also disclosed by Mihailov et al. in U.S. Pat. No. 6,993,221, in which high-order mode Bragg grating structures were proposed. This method was further extended so as to allow for the suppression of cladding mode losses, as disclosed by Mihailov et al. in U.S. Pat. No. 7,031,571. These prior art methods offer useful functions but all suffer from a practical limitation. Indeed, although it is mentioned in these patents that the method could be successfully applied to any at least partially transmissive or absorbing material, the corresponding results and examples were restricted to silica glasses only. Since low phonon energy glasses have significantly different physical, and especially thermal, properties compared to silica-based glasses, it was demonstrated that the previous femtosecond approach could not be applied, as such, to low phonon energy glasses such as fluoride, chalcogenide and chalcohalide-based glasses without significant improvements in order to obtain strong and permanent refractive index changes.
There is therefore a need for a method and system to write permanent Bragg gratings or the like in fluoride glass which can provide a strong refractive index change and can be used in a variety of fluoride glass compositions.